1. Field of the Invention
The present invention relates to an X-ray CT apparatus that can produce a desired scanogram without a missed portion (an image for setting a scanning range), although a detector is asymmetrically positioned with respect to a line connecting the focus and the rotational center of an X-ray tube.
2. Description of the Related Art
In general, when the inside of an object is imaged by an X-ray CT apparatus, first, the X-ray CT apparatus produces an image for setting a scanning range (e.g. JP-A-192327) . The image for setting a scanning range is a scanogram, i.e. a radioscopic image for a predetermined range of the object. In the imaging of the inside of the object, a position where a tomogram is imaged is determined on the basis of the scanogram. Accordingly, the X-ray CT apparatus obtains a tomogram by scanning the determined position using X-rays. When obtaining a scanogram, the X-ray CT apparatus irradiates X-rays onto the object, while not rotating the X-ray tube and moving a top board of a bed where the object is placed in the body axial direction of the object. A scanogram 30 as shown in FIG. 1 is produced on the basis of projection data obtained by the X-ray irradiation and displayed on a screen of a display.
In the determination of positions for imaging a tomogram, the positions for imaging a tomogram are set on the scanogram. The set positions are, for example, indicated by lines L1, L2, . . . Ln as shown in FIG. 1. During scanning for obtaining a tomogram, the top board of the bed where the object is placed returns to the initial position after the imaging for producing a scanogram is finished and moves again thereafter. The X-ray tube is disposed to the above positions L1 to Ln by moving the top board of the bed. The X-ray tube irradiates X-rays, rotating around the object. Tomograms for the object at the positions L1 to Ln are obtained as images based on the projection data obtained by the X-ray irradiation.
In recent years, some of X-ray detectors for X-ray CT apparatuses tend to be asymmetrically disposed with respect to an irradiation axis of an X-ray extending from the focus of the X-ray tube through the rotational centers of the X-ray tube and X-ray detector. The asymmetrically disposed X-ray detector is formed such that the right and left thereof are asymmetric with respect to the irradiation axis of the X-ray irradiated from the X-ray tube, and the edges are separated into an end closer to and the other end farther from the irradiation axis.
However, when the scanogram is imaged the an X-ray CT apparatus having an asymmetrically disposed X-ray detector, because the right and the left of the X-ray detector collecting the X-rays transmitted through the object are asymmetrical to each other with respect to the irradiation axis, a missed portion of the imaging region of the object is generated in a collection FOV (Field Of View).